/* Grammar reduction for Bison.
   Copyright (C) 1988, 1989 Free Software Foundation, Inc.

This file is part of Bison, the GNU Compiler Compiler.

Bison is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.

Bison is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with Bison; see the file COPYING.  If not, write to
the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.  */




/*
 * Reduce the grammar:  Find and eliminate unreachable terminals,
 * nonterminals, and productions.  David S. Bakin.
 */



/*
 * Don't eliminate unreachable terminals:  They may be used by the user's
 * parser.
 */



#include <stdio.h>
#include "system.h"
#include "files.h"
#include "gram.h"
#include "machine.h"
#include "new.h"


extern char   **tags;  /* reader.c */
extern int      verboseflag; /* getargs.c */
static int      statisticsflag; /* XXXXXXX */

#ifndef TRUE
#define TRUE (1)
#define FALSE (0)
#endif
typedef int bool;
typedef unsigned *BSet;
typedef short  *rule;


/*
 * N is set of all nonterminals which are not useless.  P is set of all rules
 * which have no useless nonterminals in their RHS.  V is the set of all
 * accessible symbols.
 */



static BSet     N, P, V, V1;

static int      nuseful_productions, nuseless_productions,
nuseful_nonterminals, nuseless_nonterminals;


static void useless_nonterminals();
static void inaccessable_symbols();
static void reduce_grammar_tables();
static void print_results();
static void print_notices();
void dump_grammar();

//extern void fatals ();


bool
bits_equal (L, R, n)
BSet L;
BSet R;
int n;
{
    int i;

    for (i = n - 1; i >= 0; i--)
    if (L[i] != R[i])
        return FALSE;
    return TRUE;
}


int
nbits (i)
unsigned i;
{
    int count = 0;

    while (i != 0) {
        i ^= (i & (~i + 1u));
        ++count;
    }
    return count;
}


int
bits_size (S, n)
BSet S;
int n;
{
    int i, count = 0;

    for (i = n - 1; i >= 0; i--)
    count += nbits(S[i]);
    return count;
}

void
reduce_grammar ()
{
    bool reduced;

/* Allocate the global sets used to compute the reduced grammar */

    N = NEW2(WORDSIZE(nvars), unsigned);
    P = NEW2(WORDSIZE(nrules + 1), unsigned);
    V = NEW2(WORDSIZE(nsyms), unsigned);
    V1 = NEW2(WORDSIZE(nsyms), unsigned);

    useless_nonterminals();
    inaccessable_symbols();

    reduced = (bool) (nuseless_nonterminals + nuseless_productions > 0);

    if (verboseflag)
        print_results();

    if (reduced == FALSE)
        goto done_reducing;

    print_notices();

    if (!BITISSET(N, start_symbol - ntokens))
        fatals("Start symbol %s does not derive any sentence.",
            tags[start_symbol]);

    reduce_grammar_tables();
/* if (verboseflag) {
     fprintf(foutput, "REDUCED GRAMMAR\n\n");
     dump_grammar();
     }
     */



/**/ statisticsflag = FALSE; /* someday getopts should handle this */
    if (statisticsflag == TRUE)
        fprintf(stderr,
            "reduced %s defines %d terminal%s, %d nonterminal%s\
                , and %d production%s.\n", infile,
                ntokens, (ntokens == 1 ? "" : "s"),
                nvars,   (nvars   == 1 ? "" : "s"),
                nrules,  (nrules  == 1 ? "" : "s"));

done_reducing:

/* Free the global sets used to compute the reduced grammar */

    FREE(N);
    FREE(V);
    FREE(P);

}

/*
 * Another way to do this would be with a set for each production and then do
 * subset tests against N, but even for the C grammar the whole reducing
 * process takes only 2 seconds on my 8Mhz AT.
 */



static bool
useful_production (i, N)
int  i;
BSet N;
{
    rule  r;
    short n;

/*
   * A production is useful if all of the nonterminals in its RHS
   * appear in the set of useful nonterminals.
   */



    for (r = &ritem[rrhs[i]]; *r > 0; r++)
    if (ISVAR(n = *r))
        if (!BITISSET(N, n - ntokens))
            return FALSE;
    return TRUE;
}


/* Remember that rules are 1-origin, symbols are 0-origin. */

static void
useless_nonterminals ()
{
    BSet Np, Ns;
    int  i, n;

/*
   * N is set as built.  Np is set being built this iteration. P is set
   * of all productions which have a RHS all in N.
   */



    Np = NEW2(WORDSIZE(nvars), unsigned);

/*
   * The set being computed is a set of nonterminals which can derive
   * the empty string or strings consisting of all terminals. At each
   * iteration a nonterminal is added to the set if there is a
   * production with that nonterminal as its LHS for which all the
   * nonterminals in its RHS are already in the set.  Iterate until the
   * set being computed remains unchanged.  Any nonterminals not in the
   * set at that point are useless in that they will never be used in
   * deriving a sentence of the language.
   * 
   * This iteration doesn't use any special traversal over the
   * productions.  A set is kept of all productions for which all the
   * nonterminals in the RHS are in useful.  Only productions not in
   * this set are scanned on each iteration.  At the end, this set is
   * saved to be used when finding useful productions: only productions
   * in this set will appear in the final grammar.
   */



    n = 0;
    while (1)
    {
        for (i = WORDSIZE(nvars) - 1; i >= 0; i--)
        Np[i] = N[i];
        for (i = 1; i <= nrules; i++)
        {
            if (!BITISSET(P, i))
            {
                if (useful_production(i, N))
                {
                    SETBIT(Np, rlhs[i] - ntokens);
                    SETBIT(P, i);
                }
            }
        }
        if (bits_equal(N, Np, WORDSIZE(nvars)))
            break;
        Ns = Np;
        Np = N;
        N = Ns;
    }
    FREE(N);
    N = Np;
}

static void
inaccessable_symbols ()
{
    BSet  Vp, Vs, Pp;
    int   i, n;
    short t;
    rule  r;

/*
   * Find out which productions are reachable and which symbols are
   * used.  Starting with an empty set of productions and a set of
   * symbols which only has the start symbol in it, iterate over all
   * productions until the set of productions remains unchanged for an
   * iteration.  For each production which has a LHS in the set of
   * reachable symbols, add the production to the set of reachable
   * productions, and add all of the nonterminals in the RHS of the
   * production to the set of reachable symbols.
   * 
   * Consider only the (partially) reduced grammar which has only
   * nonterminals in N and productions in P.
   * 
   * The result is the set P of productions in the reduced grammar, and
   * the set V of symbols in the reduced grammar.
   * 
   * Although this algorithm also computes the set of terminals which are
   * reachable, no terminal will be deleted from the grammar. Some
   * terminals might not be in the grammar but might be generated by
   * semantic routines, and so the user might want them available with
   * specified numbers.  (Is this true?)  However, the nonreachable
   * terminals are printed (if running in verbose mode) so that the user
   * can know.
   */



    Vp = NEW2(WORDSIZE(nsyms), unsigned);
    Pp = NEW2(WORDSIZE(nrules + 1), unsigned);

/* If the start symbol isn't useful, then nothing will be useful. */
    if (!BITISSET(N, start_symbol - ntokens))
        goto end_iteration;

    SETBIT(V, start_symbol);

    n = 0;
    while (1)
    {
        for (i = WORDSIZE(nsyms) - 1; i >= 0; i--)
        Vp[i] = V[i];
        for (i = 1; i <= nrules; i++)
        {
            if (!BITISSET(Pp, i) && BITISSET(P, i) &&
                BITISSET(V, rlhs[i]))
            {
                for (r = &ritem[rrhs[i]]; *r >= 0; r++)
                {
                    if (ISTOKEN(t = *r)
                        || BITISSET(N, t - ntokens))
                    {
                        SETBIT(Vp, t);
                    }
                }
                SETBIT(Pp, i);
            }
        }
        if (bits_equal(V, Vp, WORDSIZE(nsyms)))
        {
            break;
        }
        Vs = Vp;
        Vp = V;
        V = Vs;
    }
end_iteration:

    FREE(V);
    V = Vp;

/* Tokens 0, 1, and 2 are internal to Bison.  Consider them useful. */
    SETBIT(V, 0);   /* end-of-input token */
    SETBIT(V, 1);   /* error token */
    SETBIT(V, 2);   /* some undefined token */

    FREE(P);
    P = Pp;

    nuseful_productions = bits_size(P, WORDSIZE(nrules + 1));
    nuseless_productions = nrules - nuseful_productions;

    nuseful_nonterminals = 0;
    for (i = ntokens; i < nsyms; i++)
    if (BITISSET(V, i))
        nuseful_nonterminals++;
    nuseless_nonterminals = nvars - nuseful_nonterminals;

/* A token that was used in %prec should not be warned about.  */
    for (i = 1; i < nrules; i++)
    if (rprecsym[i] != 0)
        SETBIT(V1, rprecsym[i]);
}

static void
reduce_grammar_tables ()
{
/* This is turned off because we would need to change the numbers
   in the case statements in the actions file.  */


#if 0
/* remove useless productions */
    if (nuseless_productions > 0)
    {
        short np, pn, ni, pi;

        np = 0;
        ni = 0;
        for (pn = 1; pn <= nrules; pn++)
        {
            if (BITISSET(P, pn))
            {
                np++;
                if (pn != np)
                {
                    rlhs[np] = rlhs[pn];
                    rline[np] = rline[pn];
                    rprec[np] = rprec[pn];
                    rassoc[np] = rassoc[pn];
                    rrhs[np] = rrhs[pn];
                    if (rrhs[np] != ni)
                    {
                        pi = rrhs[np];
                        rrhs[np] = ni;
                        while (ritem[pi] >= 0)
                            ritem[ni++] = ritem[pi++];
                        ritem[ni++] = -np;
                    }
                } else {
                    while (ritem[ni++] >= 0);
                }
            }
        }
        ritem[ni] = 0;
        nrules -= nuseless_productions;
        nitems = ni;

/*
       * Is it worth it to reduce the amount of memory for the
       * grammar? Probably not.
       */



    }
#endif /* 0 */
/* Disable useless productions,
     since they may contain useless nonterms
     that would get mapped below to -1 and confuse everyone.  */


    if (nuseless_productions > 0)
    {
        int pn;

        for (pn = 1; pn <= nrules; pn++)
        {
            if (!BITISSET(P, pn))
            {
                rlhs[pn] = -1;
            }
        }
    }

/* remove useless symbols */
    if (nuseless_nonterminals > 0)
    {

        int    i, n;
/*      short  j; JF unused */
        short *nontermmap;
        rule   r;

/*
       * create a map of nonterminal number to new nonterminal
       * number. -1 in the map means it was useless and is being
       * eliminated.
       */



        nontermmap = NEW2(nvars, short) - ntokens;
        for (i = ntokens; i < nsyms; i++)
        nontermmap[i] = -1;

        n = ntokens;
        for (i = ntokens; i < nsyms; i++)
        if (BITISSET(V, i))
            nontermmap[i] = n++;

/* Shuffle elements of tables indexed by symbol number.  */

        for (i = ntokens; i < nsyms; i++)
        {
            n = nontermmap[i];
            if (n >= 0)
            {
                sassoc[n] = sassoc[i];
                sprec[n] = sprec[i];
                tags[n] = tags[i];
            } else {
                free(tags[i]);
            }
        }

/* Replace all symbol numbers in valid data structures.  */

        for (i = 1; i <= nrules; i++)
        {
/* Ignore the rules disabled above.  */
            if (rlhs[i] >= 0)
                rlhs[i] = nontermmap[rlhs[i]];
            if (ISVAR (rprecsym[i]))
/* Can this happen?  */
                rprecsym[i] = nontermmap[rprecsym[i]];
        }

        for (r = ritem; *r; r++)
        if (ISVAR(*r))
            *r = nontermmap[*r];

        start_symbol = nontermmap[start_symbol];

        nsyms -= nuseless_nonterminals;
        nvars -= nuseless_nonterminals;

        free(&nontermmap[ntokens]);
    }
}

static void
print_results ()
{
    int   i;
/*  short j; JF unused */
    rule  r;
    bool  b;

    if (nuseless_nonterminals > 0)
    {
        fprintf(foutput, "Useless nonterminals:\n\n");
        for (i = ntokens; i < nsyms; i++)
        if (!BITISSET(V, i))
            fprintf(foutput, "   %s\n", tags[i]);
    }
    b = FALSE;
    for (i = 0; i < ntokens; i++)
    {
        if (!BITISSET(V, i) && !BITISSET(V1, i))
        {
            if (!b)
            {
                fprintf(foutput, "\n\nTerminals which are not used:\n\n");
                b = TRUE;
            }
            fprintf(foutput, "   %s\n", tags[i]);
        }
    }

    if (nuseless_productions > 0)
    {
        fprintf(foutput, "\n\nUseless rules:\n\n");
        for (i = 1; i <= nrules; i++)
        {
            if (!BITISSET(P, i))
            {
                fprintf(foutput, "#%-4d  ", i);
                fprintf(foutput, "%s :\t", tags[rlhs[i]]);
                for (r = &ritem[rrhs[i]]; *r >= 0; r++)
                {
                    fprintf(foutput, " %s", tags[*r]);
                }
                fprintf(foutput, ";\n");
            }
        }
    }
    if (nuseless_nonterminals > 0 || nuseless_productions > 0 || b)
        fprintf(foutput, "\n\n");
}

void
dump_grammar ()
{
    int i;
    rule r;

    fprintf(foutput,
        "ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n",
        ntokens, nvars, nsyms, nrules, nitems);
    fprintf(foutput, "Variables\n---------\n\n");
    fprintf(foutput, "Value  Sprec    Sassoc    Tag\n");
    for (i = ntokens; i < nsyms; i++)
    fprintf(foutput, "%5d  %5d  %5d  %s\n",
        i, sprec[i], sassoc[i], tags[i]);
    fprintf(foutput, "\n\n");
    fprintf(foutput, "Rules\n-----\n\n");
    for (i = 1; i <= nrules; i++)
    {
        fprintf(foutput, "%-5d(%5d%5d)%5d : (@%-5d)",
            i, rprec[i], rassoc[i], rlhs[i], rrhs[i]);
        for (r = &ritem[rrhs[i]]; *r > 0; r++)
        fprintf(foutput, "%5d", *r);
        fprintf(foutput, " [%d]\n", -(*r));
    }
    fprintf(foutput, "\n\n");
    fprintf(foutput, "Rules interpreted\n-----------------\n\n");
    for (i = 1; i <= nrules; i++)
    {
        fprintf(foutput, "%-5d  %s :", i, tags[rlhs[i]]);
        for (r = &ritem[rrhs[i]]; *r > 0; r++)
        fprintf(foutput, " %s", tags[*r]);
        fprintf(foutput, "\n");
    }
    fprintf(foutput, "\n\n");
}


static void
print_notices ()
{
    extern int fixed_outfiles;

    if (fixed_outfiles && nuseless_productions)
        fprintf(stderr, "%d rules never reduced\n", nuseless_productions);

    fprintf(stderr, "%s contains ", infile);

    if (nuseless_nonterminals > 0)
    {
        fprintf(stderr, "%d useless nonterminal%s",
            nuseless_nonterminals,
            (nuseless_nonterminals == 1 ? "" : "s"));
    }
    if (nuseless_nonterminals > 0 && nuseless_productions > 0)
        fprintf(stderr, " and ");

    if (nuseless_productions > 0)
    {
        fprintf(stderr, "%d useless rule%s",
            nuseless_productions,
            (nuseless_productions == 1 ? "" : "s"));
    }
    fprintf(stderr, ".\n");
    fflush(stderr);
}
